The present invention relates to the manufacture of pigment granules, for example iron oxide and chromium oxide pigments.
Metal oxides, such as iron oxides or chromium oxides, are used in the pigmentation of, among other things, cement and concrete products (e.g. paving slabs and blocks), paints, plastics, toners and inks, chelants, catalysts, and also in a variety of magnetic, medical, and pharmaceutical applications. Such metal oxide pigments have traditionally been used in the form of a powder.
Powdered metal oxide pigments, such as iron oxide and chromium oxide pigments, are dusty, giving rise to health hazards and making storage and handling difficult. Also, the powders are not free flowing and so cannot readily be conveyed through pipes, which readily become blocked by the powder; furthermore the poor flowing properties of powders makes it hard to meter them using auger screws to ensure the correct proportion of pigment to base material (e.g. concrete).
Similar problems are known in other industries, e.g. in the animal feedstuff industry, and such problems have been solved to a substantial extent by granulating the product. It is readily apparent that such solutions can be applied to the field of pigments to solve the above problems. For example, it has been proposed in FR-A-2 450 273 to granulate carbon black pigment used in the pigmentation of paper and cement and concrete; here it should be understood that carbon black gives rise to an even greater dusting problem than iron oxides since the granule size of carbon black powders is much smaller than that of iron oxide powders but also carbon black suffers from an additional problem of floating on the base material, which makes incorporation into the base material difficult. According to FR-A-2 450 273, the twin problems of dusting and poor incorporation are solved by mixing carbon black with at least 30% water and optionally also a wetting or dispersing agent in an amount of 0.5 to 12% and preferably 5 to 10% (based on the amount of the carbon black) and subjecting the resulting mixture to compression forces in a pearlising machine to form pearls or granules. Depending on the nature and operation of the pearlising machine, the compression forces can be substantial.
In contrast to FR-A-2 450 273, EP-B-0 268 645 requires that no compression forces are applied to pigments during the formation of pigment granules for use in colouring of concrete and cement. This may be achieved by an agglomeration technique, e.g. by means of rotating pan or drum granulising machines, which merely bring individual pigment particles into contact with each other in the presence of water and a binder (e.g. lignin sulphonate), whereupon the particles adhere to each other, i.e. they coalesce, to form the required granules. Alternatively pigment granules may be formed by spray drying a mixture of the pigment, water and a binder and commercially it is the spray drying method that is used. Both methods, however, require the presence of a considerable amount of binders to ensure that the pigment particles adhere to one another. If made by pan or drum pelletising machines, it may be necessary to dry the granules to a commercially acceptable water content below 4.2% water.
In U.S. Pat. No. 4,277,288, it has been proposed to manufacture pigment granules by forming a fluidised bed of pigment powder and adding into the bed an organic liquid or wax as a binder to promote granulation. A surfactant is also added.
U.S. Pat. No. 5,484,481 discloses a process for the granulation of pigments for use in dyeing cement and concrete involving compacting pigment powders in the presence of a binder to form flakes, breaking up the flakes and pelletising the ground flakes using known techniques, e.g. using rotating pans or drums, which would involve the application of water and a binder to the ground flakes.
However, the granulation of pigments must meet another criterion not required in other industries where pelletisation is common, e.g. the animal feed stuff industry, namely the requirement that any pigment granules must be capable of being readily dispersed in the base material to colour it uniformly since if they did not readily disperse, they would give rise to streaks or pockets of colour, which detract from the appearance of the final product. Thus granules should be able to be dispersed in the base material while at the same time should be sufficiently coherent and robust that they do not break down into powder again during storage or handling.
The manufactures of coatings (whether liquid or dry) require that pigments contain as few unnecessary additives as possible and it would therefore be desirable to be able to produce pigments with substantially reduced amounts of binders and, if possible, even to eliminate such additives.
It has generally been thought indispensable commercially to use one or more binders (other than water or other material that is or can be removed after the formation of the granule) in the manufacture of pigment granules to give the granules strength to resist being broken up into powder during handling and storage and to promote the dispersion of these granules in their end use.
It is an object of the present invention to manufacture pigment granules that both readily disperse in the base medium and also are robust and have a reduced liability to dusting, i.e. to being broken down into powder. It is a further object of the present invention to provide a process of manufacturing robust and readily dispersible pigment granules without the use of substantial quantities of binder.
According to the present invention, there is provided a process for the preparation of low dusting, free flowing granules of at least one pigment, said at least one pigment being selected from the group consisting of iron oxides, chromium oxides, cobalt blues, mixed metal oxides, carbon blacks, titanium oxides, or mixtures thereof, which process comprises mixing said at least one pigment with water to form a mixture having a dough-like consistency, extruding the mixture through at least one die to form extruded granules, thereby also compacting the mixture, and drying the extruded granules, so that the final water content of the granules is less than substantially 5%.
The action of forcing the material through a die during the extrusion process exerts a substantial compaction on the individual pigment particles, thereby increasing the strength of the granules.
Surfactants and/or binders may be added to the extrusion dough, although any binder used is preferably of the type that also has some surfactant properties. Examples of suitable binders/dispersants are Borresperse NA, Ultrazine NA, Pexol 2000, Dresinate 214, Dispex N40, Narlex LD31, Suparex DP CC002. Surfactants (e.g. anti-flocculants or wetting agents), such as sodium alkylbenzene sulphonates, also make suitable additives, as they can provide some incidental binding action, as well as improving the dispersion properties in the end use.
The water content of the dough mixture is critical to:
forming a stable granule
preventing the extruded granules from fusing to one another
producing discrete granules rather than just a long ribbon.
but the optimum water content can readily be determined for any pigment composition by simple trial and error.
The damp mixture is fed to a compression device whereby the mixture is forced through holes in a die, which is preferably a perforated plate or screen. This can be achieved by the action of a screw pushing the mixture through the die or by the action of a moving blade or a roller (or similar pushing device) wiped over the die and thereby compressing the mixture through the die.
Typically the extruder holes would be between 0.3 mm and 4 mn in diameter, but could be smaller or larger.
The extruded granules are dried (e.g. in a tray drier, band dryer, fluidised bed dryer etc) and may then be screened to remove fines and/or oversized granules, which latter can arise either because they are too long or because individual granules have fused together. Both the fines and the oversize can be recycled, although the latter could be mechanically reduced in size and rescreened.
The shape of the granules can be further enhanced by rounding either before or after drying, which would give them a higher impact strength (and therefore a reduced liability to form dust) and a greater ability to flow.
The granules can be obtained in very high yields (e.g. in excess of 95%) and the process can easily be operated continuously and, if appropriate, automated.
The screened dried granules are relatively free of dust and fines, which is not the case with briquetted and spray dried granules. The extruded granules are low dusting, robust and exhibit good controllable flowability and handling properties.
The extruded granules of the present invention have greater impact strength than briquetted granules when made to have similar ability to be redispersed in the end use, e.g. in concrete. Looked at another way, the extruded granules having similar redispersion properties to briquetted granules have a greater impact strength. Thus, in general the redispersion properties and impact strength of the extruded granules are superior to spray dried granules.
The quantity of binder/surfactant used can be very low and indeed it is possible to dispense with such additives altogether, which is extremely advantageous for pigments used in wet or dry coatings industries (e.g. paints), where such additives are highly disadvantageous. This is a distinct advantage over spray dried and briquetted granules, where high levels of binders and/or surfactants are required.
The shear forces exerted and the mechanical energy input for the granule formation (and hence the compaction exerted on the pigment during granule formation) can be adjusted by:
changing the extrusion hole size (the larger the diameter, the lower the shear)
changing the extrusion speed, (e.g. the speed of the wiper blade/roller or the feed screw (the slower the speed, the lower the shear).
The compaction exerted on the pigment during granule formation brought about by the shear force and mechanical energy input during extrusion will determine the granules"" redispersion and strength properties and hence by suitably setting these parameters during the manufacture of the extruded granules, the properties of the granules can be adjusted to match their intended end use. For example, in uses where redistribution is not a problem, high shear forces can be used during manufacture, which will mean that the granules will have high impact strengths and a low propensity to form dust during storage and handling. However, where easy redistribution properties are required, low shear forces should be used, but this will also make the granules less strong.
The invention will be illustrated by a number of non-limiting Examples. In the Examples, the percentages stated are by weight based on the weight of the pigment used.
In the present Examples, granules were subjected to various tests which were all conducted in the same manner:
Yield Test
The granules were screened and the percentage of granules produced having a diameter in the range 0.5-2.4 mm was measured together with the percentage of oversized granules having a diameter greater than 2.4 mm.
Flow Rate Test
The time taken for 100 g of granules to flow through a funnel having a 15 mm diameter aperture from a static start was measured.
Drop Strength
A sample of granules is sieved to remove fines (which are, except when specified otherwise,  less than 0.5 mm) and the granules were then dropped from a height of 750 mm onto a steel plate tilted at an angle of 45 degrees. The dropped sample is then sieved again and the fines ( less than 0.5 mm) generated by the drop are expressed as a percentage of the total sample weight. Hence the lower the fines generation figure, the higher the granule/granule impact strength.
Colour Shift (Delta E)
The colour shift displayed by a concrete brick made using the granular pigment as compared to a standard brick made using the original pigment powder was measured. The target Delta E should be less than 2.
Bulk Density
The bulk density of granules is measured by taking a known volume of granules in a bottle and weighing the bottle. The weight of the bottle is subtracted and the bulk density can then be calculated expressed as g/cc.
Dispersion Test
A weighed sample of material is stirred at a fixed rate in water for a fixed time, e.g. 3 grams in 225 cc of water, stirred for 5 mins using a 50 mm straight bladed turbine laboratory mixer at 1720 rpm (tip speed 4.5 m/s). The resulting slurry is then wet sieved through a 63 micron screen and the retained residue is dried, weighed and expressed as a percentage of the initial sample weight. The lower the residue figure, the more easily the material will disperse in the end use.